1. Field of the Invention
The present invention relates generally to dross separation methods, and more particularly, to a dross separation method applied in a soldering process for a printed circuit board.
2. Description of Related Art
In the manufacturing process of a printed circuit board (PCB), a soldering process plays a key role in determining the quality of the printed circuit board manufactured, which is conventionally performed in an automatic wave soldering furnace.
The automatic wave soldering furnace essentially comprises a conveyor belt, a flux adding zone, a preheating zone and a tin bath. The conveyor belt is used for conveying a printed circuit board into the automatic wave soldering furnace, passing through the flux adding zone, the preheating zone and the tin bath. The flux adding zone essentially comprises an infrared sensor and a nozzle, wherein the infrared sensor senses whether a printed circuit board has come into the furnace and measures the length of the printed circuit board. The flux is used for forming a protective film on the soldering surface of the printed circuit board. The preheating zone is used to provide a suitable temperature to enhance the flux activity so as to form good solder joints. Heating pipes and a tin pump are disposed in the tin bath such that molten solder (liquid tin formed after tin is melted) is sprayed upwards to form a projected solder wave. During the soldering process, a printed circuit board with components placed thereon is coated with flux, preheated and passed across the solder wave so as to solder the components to solder pads of the printed circuit board.
In a practical wave soldering process, after tin in a tin bath is melted, high-temperature tin on the surface of the molten tin is in direct contact with air. Particularly, the tin at the nozzle of the tin bath continuously rolls and is in direct contact with the air, thereby producing some dross (oxide of tin) floating on the liquid tin. If the dross is not removed timely, solder falling back from the solder wave will fall on the surface of the dross. Since the falling solder is in a semi-solidified state, it is easy to flow out of the tin bath, which is both wasteful and dangerous. Therefore, it is extremely important to timely remove the dross.
Generally, the dross is scooped up and collected and then disposed in a recovery machine to recover pure tin from the dross. However, the recovery machine is quite expensive and susceptible to serious heavy metal pollution, and thus the recovery machine requires peripheral equipments to prevent serious heavy metal pollution, which is not cost-effective for manufacturers. Therefore, the dross is usually sold to scrap dealers. In the practical soldering process, the dross floating on the liquid tin needs to be periodically removed so as to prevent overflow of the liquid tin, thereby increasing the burden on operators. Since the dross has to be frequently scooped away, it is necessary to add new solder material to the tin bath to replenish the tin bath, which accordingly increases the solder consumption. A method for reducing the generation of dross involves adding a reducing powder in the tin bath. But the reducing powder bring serious pollution to the tin bath and is not easy to clean, and the number of times for the replacement of tin increases with the replacement of the reducing powder. Therefore, the method is seldom used.
To overcome the above drawbacks, an antioxidant oil is introduced over the liquid solder to isolate the liquid solder from the air so as to minimize oxidation of the solder, as the antioxidant oil floats on the liquid tin. However, the oil sludge produced during the use of the antioxidant oil pollutes the tin bath and produces smoke that must be dissipated by a ventilation device. Further, the produced oil sludge is easy to mix with the dross, which adversely affecting the recycling of the dross.
Therefore, it is imperative to provide a simple method to facilitate the dross separation and reduces the loss of tin so as to reduce tin consumption in the soldering process.